1. Field of the Invention
The present invention relates to a gearbox for use with a speed reducer or the like.
2. Description of the Related Art
Various speed reducers have been conventionally used in power transmission systems for driving, industrial equipment in factories. In general, the speed reducer includes a gear transmission mechanism of a particular structure within a predetermined gearbox. In the gear transmission mechanism, the gears are meshed with each other, thereby periodic vibrations which depend on the number of teeth and the rotational speed are generated due to collision of tooth surfaces and slight deformation of the tooth surfaces, so that the so-called gear noise develops. Once the center frequency of the gear noise has become equivalent to the natural frequency of each member in the speed reducer, especially, that of the gearbox, or to an integral multiple of the natural frequency thereof, the noise might be further amplified because of a resonance phenomenon.
In recent years, efforts have been made to improve working environments such as in factories. This trend has brought about a serious challenge to reduction of noise that is produced by the speed reducer.
The gearbox is larger in size than any other members in the speed reducer, with its wall surfaces more likely to amplify vibrations. Therefore, the gearbox has a large influence on noise generated by the whole speed reducer.
To address these problems, a structure with a gearbox having an increased-thickness portion at the center of its walls is suggested, e.g., in Japanese Patent Laid-Open Publication No. 1975-125159.
However, any conventional countermeasures of this type against noise are based on the concept of enhancing the rigidity of the gearbox, and have not been always successfully taken in practice to effectively reduce the noise.
The gearbox is classified into two types: one type with an input shaft and an output shaft arranged coaxially, and the other type with an input shaft and an output shaft arranged non-coaxially. Of these types, in particular, the gearbox with an input shaft and an output shaft arranged non-coaxially has a larger number of shaft holes, in each of which disposed is a shaft acting as a vibration source. This structure thus raised a problem that vibration inducing force on each shaft is transmitted to the entire gearbox, thereby causing resonance to more likely occur.